The present invention relates to an image processor and an image processing method, and more particularly, relates to an image processor and an image processing method for decompressing progressive-coded compressed image data.
Still image data exchanged on the Internet includes compressed image data subjected to sequential coding (sequential-coded compressed image data) and compressed image data subjected to progressive coding (progressive-coded compressed image data). In normal, terminals handling still image data exchanged on the Internet, such as personal computers (PC), are provided with an image processor capable of decompressing not only sequential-coded compressed image data but also progressive-coded compressed image data.
An image processor decompresses sequential-coded compressed image data for each block. Therefore, a final image is gradually displayed on a screen starting from the upper left of the screen to the lower right.
Progressive-coded compressed image data is decompressed in the following manner. First, coded data at the first stage among the compressed image data is decompressed. This provides a rough entire image with low resolution and gray scaling. Subsequently, coded data at the second stage among the compressed image data is decompressed, and the resultant decompressed image data is superposed on the already-obtained first-stage decompressed image data. This provides decompressed image data with resolution and gray scaling improved from those of the first-stage decompressed image data. Thereafter, the series of processing of decompressing coded data at each stage and superposing newly decompressed image data on the already-obtained decompressed image data are repeated up to final-stage coded data. As a result, in decompression of progressive-coded compressed image data, one rough entire image with low resolution and gray scaling is first displayed on a screen, and the entire image is improved in resolution and gray scaling step by step.
In the conventional image processor described above, for decompression of progressive-coded compressed image data, a memory is required for storing decompressed image data obtained by decompressing coded data at each stage. In general, the memory capacity required for storing decompressed image data obtained by decompressing compressed image data is ten times as large as the memory capacity required for storing the compressed image data.
With the recent rapid spread of the Internet, not only PCs but also small-size, lightweight portable terminals such as cellular phones now handle still image data. To ensure reduction in the size and weight of such portable terminals, the capacity of a memory required for an image processor mounted therein must be as small as possible.